As mentioned above, thick steel materials represented by thick plates are used in various fields for improving properties such as high strength, high toughness and the like. Recently, it is demanded that these properties are uniform in the thickness direction of the steel material and the scattering of these properties between the steel materials is less.
For example, it is reported in pages 11 to 21 of "Tetsu-to-Hagane", Vol. 74(1988), No. 6 that the destruction of high-rise buildings is designed to be controlled by absorbing vibration energy through the deformation of the building against big earthquake with the heightening of the building. Concretely, the framework of the building is fallen down at a given form in the occurrence of earthquake and hence the deconstruction of the building is controlled by the plasticization of the framework. That is, assuming that the framework of the building indicates a behavior aimed by the designer in the occurrence of earthquake, it is required that the proof ratio of the steel material used in pillar, beam and the like of the building is completely grasped by the designer. Therefore, it is inevitable that the steel materials such as plates, H-sections and the like for use in the pillar, beam and the like are homogeneous, and also the scattering of strength in the steel material becomes a great problem.
Since the steel materials used in the buildings and shipbuildings are required to have high tension and toughness, these materials are usually produced by controlled rolling and controlled cooling process or so-called TMCP process. However, if thick steel materials are produced by the TMCP process, the cooling rate differs in the thickness direction or between the steel materials to change the texture, and hence the scattering of properties is caused in the thickness direction of the resulting steel material or between the steel materials. The scattering of properties appears in the thickness direction of the thick steel plate, between web and flange of H-section steel because of non-uniform cooling between the web and the flange, between the lots of the steel material or the like.
In JP-A-4-224623, it is proposed that the texture in the thickness direction can be changed into a mixed texture of ferrite and bainite by adding Nb and making the cooling rate after the rolling not less than 3.degree. C./s and setting an upper limit of the cooling stop temperature at 500.degree. C., whereby the strength of a center portion in the thickness direction is increased to decrease the difference of hardness in the thickness direction. In this case, however, the cooling rate should strictly be controlled to not less than 3.degree. C./s even in the center portion, because if a cooling rate distribution is formed in the thickness direction, a scattering of properties is immediately caused. As a result, it is required to strictly control production, so that the above proposal is unsuitable for the production in industrial scale.
In JP-A-62-130215, it is proposed to improve low-temperature toughness by ensuring strength through precipitation strengthening of Cu and by cooling to 300-700.degree. C. at a cooling rate of not less than 0.5.degree. C./s after the hot rolling and holding at a temperature zone of 500-650.degree. C. for a constant time and then cooling to room temperature. However, this technique aims at the improvement of low-temperature toughness, so that it is difficult to satisfy the uniformity of the properties required in recent structural steels or the like by controlling the scattering of properties at various forms as mentioned above.